There is increasing evidence that cyclic metabolic pathways involving the key intermediate, pyruvate, are involved in a number of important biochemical processes. These include transfer of carbon and hydrogen from the mitochondria to the cytoplasm to permit biosynthesis of fatty acids and glucose. Recent experiments also suggest that a cycle involving pyruvate may be importantly involved in the mechanisms of heat generation caused by thyroid hormones. Using isotope procedures to map the metabolic pathways, and also specific inhibitors to block key enzyme reactions, we will determine the dietary and hormonal control factors which regulate the metabolism of these important systems.